I have an existing Centos5.5 build, and i want to keep data on that hard disk.

I want to install a new version of Centos or other version/flavour, so what is the best approach :

1. Backup existing data, install new version overwriting eisting ??

2. How should i partition the data on the same hard disk for any future upgrades or changes, such that this data is never overwritten - should i use a specific mount point - and what is the best installation method ?. (GUI based, or command line ???)

It depends on whether or not you have a separate /home partition, whether you use mysql and how much tinkering you have done.

If you have a separate /home partition, all your personal data will be there.

If you use mysql, you need to use mysqldump to backup your mysql databases to your /home partition.

If you tinker a lot, you probably also need to backup any files in /etc which you have modified to your /home partition.

Once you have everything you want to keep on your /home partition, back it up and proceed. Most distros will automatically leave /home untouched but it is better to be safe than sorry. (I have never had to use my backup.)

If you don't have a separate /home partition, you will need to back all these things up anyway. When you install the new version, look for an option to create a separate /home partition to make things easier in future.

(Note also that a handfull of other programs (e.g. evolution) will only restore, like mysql, from their own backup format. So back these up using their own format before you back up /home.)

So, are you suggesting backing up in case it all goes wrong, or are you indicating that backing up is required since you cannot install a different Linux flavour without the entire hard disk being affected ?.

Is this installer specific ?. so Ubuntnu is ok, but Fedora cannot since the installer is not very intelligent ?

Backups should be on a separate medium. All distro installers allow you to choose your partitioning layout, so you can reuse existing /home and /var partitions, retaining their contents. All distros (well, except Gentoo) use a GUI installer, even if it is ncurses in a console, so that question doesn't really matter - except the "proper" GUI installers are generally easier to find your way around.

"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)

Thanks - i will try the GUI with a /data and /apps mount point to see what occurs - i can backup on to another hard disk as a precaution (will be difficult due to SATA restrictions i have) but this will be a good test.